The objective of this project is to use the techniques of somatic cell genetics to analyze the events leading to the expression by the tumor cell of a new antigen on its surface. The antigens of the gp 69/71 glycoprotein of Gross MuLV are being studied as a model system. Variant cell lines with defects in the expression of particular surface antigens are to be isolated. The genes defined by these variants are then to be studied by cell hybridization to determine their number, nature, and relationship to genes defined in studies in the intact animal. Studies are also being carried out on the relationship between viral integration and antigen expression, using expression of the Thy-l antigen in virus-infected BALB 3T3 cells as a model system.